Effects of α1-and α2-adrenoceptor agonists on actionpotential and force of contraction in rabbitpapillary muscle

Effects of α1-adrenoceptor agonist phenylephrine and α2-adrenoceptor agonist B-HT 920 on action potential and force of contraction were investigated in rabbit papillary muscle.1. In the presence of propranolol (0.2 μM), phenylephrine (0.1-100 μM) prolonged action potential duration (APD50, APD90) and effective refractory period (ERP), increased force of contraction in a concentration dependent manner, but had no significant effects on resting potential (RP), overshoot potential (OP), amplitude (APA) and maximum rate of rise (dV/dt)max of action potential.2. The effects of phenylephrine (20 μM) on action potential and force of contraction were antagonized by prazosin (0.4 μM).3.In reserpinized rabbit papillary mus-cle,lower concentration of B-HT 920(1.0μM) exerted no significant effects on action potential and force of contraction; at higher concentrations (10 and 100 μM) , B-HT 920 prolonged APD50, APD90 and ERP significantly, and also had no significant effects on RP, APA, (dV/dt)max and force of contraction.4. The effects of B-HT 920 (10 μM) mentioned above could not be antagonized by yohimbine (0.1 μM).In conclusion, in rabbit papillary muscle the physiological effects of postsynaptic α-adrenoceptor stimulation are mainly mediated by α1-adrenoceptors, and our experiment could not prove the presence of functional postsynaptic α2-adrenoceptors.